can: mcp251xfd: add BQL support

This patch re-adds BQL support to the driver. Support for
netdev_xmit_more() will be added in a separate patch series.

Link: https://lore.kernel.org/r/20210406110617.1865592-3-mkl@pengutronix.de
Cc: Manivannan Sadhasivam <mani@kernel.org>
Cc: Thomas Kopp <thomas.kopp@microchip.com>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>